The invention relates to stable cationic or amphoteric (co)polymers obtained by Hofmann degradation carried out on a base (co)polymer comprising acrylamide or derivatives and incorporating, at its very heart, at least one polyfunctional compound containing at least 3 heteroatoms each having at least one mobile hydrogen, and also to the uses thereof as flocculation, retention and/or drainage aids, and dry strength aids in a paper-making process.
More specifically, according to the invention, the cationic or amphoteric acrylamide-based (co)polymers are organic polymers characterized in that they are obtained by Hofmann degradation on an acrylamide-based base (co)polymer incorporating, at its very heart, at least one polyfunctional compound containing at least 3 heteroatoms each having at least one mobile hydrogen, in the presence of an alkali and/or alkaline-earth metal hydroxide (advantageously sodium hydroxide), and of an alkali and/or alkaline-earth metal hypochlorite (advantageously sodium hypochlorite).
Hofmann Degradation
This reaction, discovered by Hofmann at the end of the nineteenth century, makes it possible to move from an amide to a primary amine having one fewer carbon atom. The reaction mechanism is given in detail below.
In the presence of a base (sodium hydroxide), a proton is removed from the amide.

The amidate ion formed then reacts with the active chlorine (Cl2) of the hypochlorite (e.g.: NaClO, which is in equilibrium: 2NaOH+Cl2⇄NaClO+NaCl+H2O) to give an N-chloramide. The base (NaOH) removes a proton from the chloramide to form an anion. The anion loses a chloride ion to form a nitrene, which undergoes a rearrangement to an isocyanate.

Via reaction between the hydroxide ion and the isocyanate, a carbamate is formed.

After decarboxylation (elimination of CO2) starting from the carbamate, a primary amine is obtained.

For the conversion of all or some of the amide functions of an acrylamide (co)polymer to amine functions, 2 main factors are involved (expressed as molar ratios). These are:—Alpha=(alkali and/or alkaline-earth metal hypohalide/acrylamide) and—Beta=(alkali and/or alkaline-earth metal hydroxide/alkali and/or alkaline-earth metal hypohalide).
The polymers obtained by the Hofmann degradation reaction are especially used in paper manufacturing processes. Generally, it is known that in order to be effective as strength aids, the (co)polymers used must be in the most cationic form possible and therefore must have a high degree of degradation. Indeed, as a function of the Alpha degree of degradation, it is possible to generate variations of cationicity linked to the amount of amine functions produced on the carbon-based backbone of the (co)polymer. Furthermore, it is known that when the product obtained by the Hofmann degradation reaction is amphoteric, this product can also be used to improve the retention of fillers during the paper or board manufacturing process, while maintaining the dry strength performances.
Until recently, only very expensive processes using, in situ, a unit for manufacturing a Hofmann degradation product (EP 377 313) or processes using another polymer (base of N-vinylformamide (co)polymer type followed by a hydrolysis), which is itself relatively costly (US 2004/118540), have been installed.
The first viable industrial solution was proposed at the beginning of 2005 in document WO 2006/075115 by the applicant. In this document, the Hofmann degradation product claimed is an organic polymer produced at a concentration of greater than 3.5% by weight and preferably greater than 4.5%. Even though the dry strength performances are greatly improved, the polymers produced have a very low molecular weight giving the (co)polymer a very limited advantage for applications such as drainage or flocculation.
Documents WO 2008/107620 and WO/2010/061082 A1 by the applicant have made it possible to partly solve this problem of low drainage performances. However, a person skilled in the art who is in need of increased drainage and retention has had to face a certain limitation of the performances of his products.
In document JP 61120807, a polyacrylamide is prepared by Hofmann degradation reaction on a polymer, in the presence of guanidinium. The guanidinium reacts with an intermediate compound of the Hofmann degradation, an isocyanate. The polymer obtained in this document is therefore a polymer modified during the Hofmann degradation, with modification of an intermediate compound.
Document WO2009/013423 A2 by the applicant describes a (co)polymer derived from acrylamide obtained by Hofmann degradation on a base (co)polymer. The (co)polymer is then branched. This document also discloses the possibility of adding certain additives (polyamines, polyethyleneimines, etc.) to the solution of base (co)polymer, before or during the Hofmann degradation reaction. It is the addition of additives capable of reacting with the isocyanate functions of the polymer that are generated during the degradation of the base (co)polymer.
The problem addressed by the invention is therefore to develop a wide range of polymers, the drainage and retention properties of which can be further improved relative to the preceding invention.
Due to their intrinsic properties (stabilization, cationization, complexation, etc.) the polymers of the invention may also be advantageous in very diverse applications, mention will be made, nonlimitingly, of water treatment (drinking water or wastewater), coagulation/flocculation techniques, the mining industry, the cosmetics and detergent industry, the textile industry, etc.